DE8533598U1 - Membrane for electro-acoustic transducers - Google Patents

Description

PATENT ATTORNEY DR.-ING. KLAUS DURM KARLSRUHE FELIX-MOTTL-STRASSE PHONE 853355 AT THE EUROPEAN PATENT OFFICE

· ■ ■ ·

B 3337/85 Gbm 21 November 1985

Bernhard Thiel _f technical employee Jahnstraße

6653 Blieskastel 5 15

Membrane for electro-acoustic transducers

&igr; ²⁰

j The innovation concerns a membrane made of a little

' stretchable and compressible material for electro-acoustic

converter.

The innovation is particularly used in membranes for loudspeakers.

A vibrating membrane for electro-acoustic transducers should be as stretchable and compressible as possible and should also be extremely lightweight. The loudspeaker membranes known to date only partially meet these requirements.

It is known to make membranes for loudspeakers out of cardboard» 35

cast, plastic and metal. Membranes made of cast cardboard or plastic are inexpensive to produce and have a high level of internal damping, but due to their low strength they tend to produce partial vibrations whose frequencies are in the middle of the hearing range and are therefore perceived as very disturbing. The internal damping of the material can mitigate these partial vibrations, but leads to other errors such as Z. &Xgr;. &Kgr;&igr;&igr;&Ggr;&Ggr;&bgr;&Pgr;

Q or poor impulse response. With membranes made of metal (aluminium, titanium, very rarely beryllium), the partial vibrations in the frequency range are higher due to their greater strength. However, they are still in the working range of the respective loudspeaker and are very loudly audible because the internal damping is only minimal.

The aim of the innovation is to design a membrane for electro-acoustic transducers made of a material with the lowest possible stretchability and compressibility, which does not have the disadvantages of known membranes.

As a solution to the problem, the innovation proposes using aluminum oxide in crystalline form, i.e. corundum, as the material for the membranes of electro-acoustic transducers.

Aluminium oxide in crystalline form has only a very low stretchability and compressibility, which means that membranes with very high stiffness can be produced. In addition, it has been shown that

t (11*1 ····*

it is possible to produce membranes of any shape, very thin and therefore extremely light, whose small mass causes only small inertial forces. As a molded body made of pure aluminum sheet that corresponds exactly to the final shape of the membrane can be used, there are no difficulties with regard to the sometimes very complicated shapes of the membranes.

The manufacture of a membrane according to the innovation is carried out by first forming a molded part from pure aluminum, which is then subjected to anodic oxidation. By firing at temperatures above 600 degrees Celsius, the aluminum oxide is converted into a crystalline modification, i.e. corundum is created, a material which gives the molded part the desired properties.

The innovation is explained in more detail below using the attached drawing. This drawing shows a simplified schematic representation:

Fig. 1 a thin-walled molded part in the form of a membrane for electro-acoustic transducers in a cross-section;

Fig. 2 the production of the molded part according to Fig. 1 by deep drawing from pure aluminum sheet;

t t

t i'i iir »lii- in

Flg. 3 Degreasing and rinsing of the

deep drawing formed part;

Fig. 4 the cleaning and etching of the degreased molded part;

Fig. 5 the neutralization of the purified

Moulded part J

Fig. 6 the anodic oxidation of the molded part

with direct current;

Fig. 7 the firing of the anodically oxidized molded part at temperatures above 600 degrees Celsius.

The thin-walled molded part 1 shown as an example in Figure 1 is a membrane for an electro-acoustic transducer, namely a loudspeaker membrane. The molded part 1 is seamlessly formed from a thin pure aluminum sheet (purity 99.95 %) with a thickness of 0.1 millimeters and consists of a circular clamping edge 2, a flat-conical funnel 3 and a cylindrical voice coil holder 4. The clamping edge 2 can have beads, which are not shown in the drawing.

The molded part 1 is produced by deep drawing, as shown in Figure 2. To do this, the lightly greased plate made of pure aluminum sheet is placed on the mold 5 and held in place at the later clamping edge 2 by the hold-down device 6. The plate is deformed into the molded part 1 using the stamp 7.

gg The thus formed by deep drawing from pure aluminium

&ngr;&igr;&igr;

The molded part 1 formed is degreased and rinsed by repeated immersion in a tank 8 containing carbon tetrachloride (CCl4) - see Figure 3.

The degreased molded part 1 is then cleaned of adhering oxide layers and dirt by placing it in a vessel 9 with sodium hydroxide solution (NaOH) and at the same time etched - see Figure 4.

The residues of the caustic soda still adhering to the molded part 1 are neutralized by introducing them into the tank 10 filled with diluted nitric acid (NO3H).

The molded part 1 prepared as described is now subjected to anodic oxidation - see Figure 6. In a trough 11 there is oxalic acid or ^B formic acid in which the molded part 1 is suspended.

In the trough 11 there is a pure aluminum plate 12 serving as a cathode. The molded part 1 forming the anode and the pure aluminum plate 12 are connected to a direct current source 15 by means of electrical lines 13 via an ammeter 14.

The electrolysis (during which hydrogen (H) is developed) is carried out in the trough 11 at a current of about 1 ampere per square decimetre of the surface of the moulded part 1 until the latter has become transparent - this is easy to observe - and the current indicated on the ammeter 14 has fallen to a hundredth, to about 10 milliamperes per square decimetre. The original pure aluminium (Al) of the moulded part 1 then becomes aluminium oxide (AI2O3) in a predominantly amorphous modification (with embedded water of crystallisation and impurities from the previous treatments with the acids).

Il It

&igr;&igr;&igr;

.+ 6 V I

As a final step, the molded part 1 consisting of amorphous aluminum oxide is placed in a kiln.

and exposed to temperatures of at least 600 | degrees Celsius until the (amorphous) aluminium [·

5 oxide (AI2O3) of the molded part 1 into a kri- j

stalline modification. The firing process then produced the hard and brittle, chemically resistant and highly insulating material corundum with a specific weight of about 3.9. Except for the shrinkage that is inevitable due to the firing, the molded part 1 now made of corundum is identical to the one made of pure aluminum.

Ct(I Î C

[ t &iacgr;&iacgr;

1 B 3337/85 Gbm

21 November 1985

List of reference numbers used

10 1 molded part • 2 Clamping edge 3 funnel 4 Voice coil holder 5 shape 25 5 Hold-down clamp 7 Rubber stamp 8th Tub 9 vessel 10 pool 20 11 trough 12 Pure aluminum plate 13 cables 14 ammeter 15 DC source 25 16 kiln

« ■ »ttt t t I t If(I '

* · * I Ii

Claims (1)

. I 1 i IH ·'· &igr; * a &igr; i I · ■ 1 Il I I · 1 B 3337/85 Gbm 21 November 1985 10 Right to protection Membrane for electro-acoustic transducers made of a material with low stretch and compression, consisting of 15 aluminum oxide in crystalline form. in it·